Circuit boards that carry integrated electronic circuits as well as discreet electronic components are well known. To properly place an integrated circuit or other component on a circuit board, the leads of the component must be aligned to corresponding pads on the circuit board within a specified tolerance. The pattern of the pads on the circuit board is determined by the function of the circuit board and is designed on the circuit board prior to assembly. In order to place a component on the board, the position of each lead must be accurately measured and a fitting algorithm must be used to match the resulting lead pattern to the known pad pattern. This information is used to produce the correct angular orientation and lateral positioning of the component as it is placed in position.
Surface mounted circuit boards can carry many types of components including those with and without leads. Integrated circuit components (ICs) usually have leads on two or four sides of a rectangular body. Typical IC types include Quad Flat Packs (QFPs), Small Outline Integrated Circuits (SOICs), Plastic Leaded Chip Carriers (PLCCs) and Thin Small Outline Packages (TSOPs). Leads can be either gull-wing shaped (QFPs, SOICs, TSOPs) or J-shaped (SOICs, PLCCs). Gull-winged leads are used to obtain a higher density of leads on a component, but J-shaped leads are more sturdy. Manufacturers are increasingly creating integrated circuits with even more delicate leads structured on tape, which are used in a process called tape automated bonding (TAB). Circuit boards can also carry leadless components, which have pads rather than leads that make contact with the circuit board, including Leadless Chip Carriers (LCCs), Ball Grid Array Chips (BGAs), Capacitors and Resistors.
The separation between centers of any pair of adjacent leads on electronic components is referred to as the pitch. Currently, a commonly manufactured lead separation is 0.025 inches (25 mil) pitch, meaning that the center of the leads are spaced at 25 thousandths of an inch intervals. Advances in component manufacturing technology, however, have produced integrated circuits having 15 and 10 mil pitches and TAB components have been created having several hundred leads spaced with a 4 mil pitch.
The bottom ends of the leads form a seating plane that will meet the plane formed by the pads on the circuit board when the component is placed in position. The height of the package body above the seating plane is called the package standoff. For some components the body extends down to the seating plane of the leads, i.e. the package standoff is essentially zero.
Solder paste is applied to the pads on the circuit board before the placement of the components. After the components are accurately placed on the solder paste, the circuit board is heated in an oven. When heated, the solder reflows and the component becomes electrically and mechanically attached to the board. Typically, solder paste is approximately 8 mils thick when applied and 4 mils thick upon cooling after reflowing. Accordingly, if one or more of the leads are bent above the nominal plane of the other leads by more than 4 mils, the bent lead likely will not connect through the solder to the circuit board, resulting in an open circuit.
The dimensions of components placed on circuit boards normally vary between 0.02 inch and 2.0 inches, although larger components may need to be accommodated. For quality manufacturing, component leads must be placed with at least 80% overlap of lead onto the corresponding pad of the circuit board. For example, a device having a 20 mil pitch generally has 10 mil wide leads. With an 80% overlap, at least 8 mils of the lead width must be on the pad with no more than 2 mils of the lead width off the pad. In general, sensing systems used to align parts for placement must have five to ten times better resolution than the accuracy required. Therefore, 0.2 to 0.4 mil image resolution is required to achieve the maximum placement error of 2 mils specified for quality manufacturing methods for a component with 20 mil pitch. Correspondingly smaller image resolution is required for components with smaller pitch.
To perform this delicate task, precision surface mount component placement machines have been developed. While the particular design of the component placement machine is not relevant, all component placement machines generally pick up a component at one location, properly orient the component and place the component in its proper location on the circuit board. The components are not precisely aligned in the component bins where they are picked up. Therefore, gull wing type components may be out of position by as much as plus or minus 50 mils and plus or minus 5 degrees angular orientation. Other type components will be similarly out of position. To obtain proper placement, the orientation and lateral position of the components from the bins must be determined and then corrected prior to placement.
In a surface mount component placement machine, a transport arm picks up the component from a component bin utilizing a vacuum quill. The vacuum gently picks up the component to be placed and transports it between the component bins and the circuit board. The transport arm moves the vacuum quill and the component from the bin to a circuit board located on a work table. Sometime during transport, the angular orientation of the component and the offset of the component from the center of the quill must be determined. During manufacturing, packaging, shipping and loading into a part bin, it is possible for the leads to get slightly bent away from their optimal position. Therefore, the condition of the leads should also be checked to determine whether any are bent or skewed.
Bending or skewing of the leads can result in an improper pitch between adjacent leads (pitch error) or in displacement of the end of a lead from the plane formed by the other leads of the component (coplanarity error). Measurements of pitch error and coplanarity error are needed to ensure that the lowest surfaces of all of the leads are in the same horizontal plane and will make contact with the proper pad on the board when the component is properly oriented. If the errors are too large, alignment of the component will not be sufficient to obtain a functioning connection with the pads of the circuit board, and an electronically defective circuit will result. Components with sufficiently bent leads should be excluded by the placement system.
For components with non-defective leads, any necessary corrections in placement are calculated and the placement head is adjusted to accommodate the corrections based on the calculated offset and angular orientation. The vacuum quill is then precisely lowered to fit the component on the circuit board. In current component placement machines, the transport arm and quill move at approximately one meter per second.
Conventional vision systems used in conjunction with component placement machines for lead position determination use solid state television cameras having a resolution of 512.times.512 picture elements or pixels. When viewing a two inch component, a corresponding two inch field of view with 512 elements produces a basic resolution of 4 mils or 4 thousandths of an inch. This is not sufficient resolution and, in fact, as pointed out above, it is necessary to achieve a resolution which is at least an order of magnitude greater. One solution is to use several cameras, but the use of several cameras is expensive.
A light based system utilizing one or more focused light sources has been proposed to accurately sense the position and condition of each of the many leads used on integrated circuits prior to their placement on a surface mount circuit board by a pick and place machine. U.S. Pat. Nos. 5,309,223 and 5,331,406, respectively entitled A Laser-Based Semiconductor Lead Measurement System and Multi-Beam Laser Sensor for Semiconductor Lead Measurements, both patents assigned to the assignee of the present invention, describes such a device. Using one, two, three or four light sources, preferably laser diodes, the sensor system can, with the highest degree of resolution, determine lateral orientation and coplanarity of leads for integrated circuit components, even those having an ultrafine pitch.
Determination of the lead position by the above mentioned method is based on the integrated circuit leads occluding the light of one or more precisely directed and focused laser light sources. Each integrated circuit lead is passed through the focal point of a laser beam. The position of each lead is determined by sensing the blockage of all or a portion of the light of the laser beam. Using a two-laser beam system, a coplanarity measurement also can be achieved to determine vertical displacement (height) of any lead.
While this system is reasonably fast, each lead must actually be moved in relation to the focal point of the one or more laser beams. To measure the leads on the four sides of a QFP component, for instance, requires multiple passes through the detection system. The lasers are relatively expensive to purchase and to operate. Furthermore, the sensor system is external to the component placement head, so that the component must be moved to the location of the sensor, which also consumes time.
U.S. Pat. No. 5,278,634, assigned to the assignee of the present invention, entitled A High Precision Component Alignment Sensor System, incorporated herein by reference, discloses a non-contact, laser-based alignment sensor located on the placement head that is utilized to generate the correct angular orientation in the X-Y plane of the component for placement and also to determine electronically any offset of the center of the component with the center of the vacuum quill which carries the component to the circuit board to allow lateral alignment of the component.
The high speed laser-based system disclosed in the '634 patent utilizes a stripe of laser light which is directed horizontally at the component whose alignment is being sensed. The shadow cast by the component is detected by a linear array detector whose data is analyzed to detect the leading edge and the trailing edge of the shadow. This shadow edge information is analyzed at various angular orientations to calculate the proper angular orientation and lateral alignment of the component. The light path cannot be significantly tilted from the horizontal without losing sensitivity.
Copending U.S. patent application Ser. No. 08/289,279, filed Aug. 11, 1994, assigned to the assignee of the present invention, (hereinafter the '279 application) entitled High Precision Component Alignment Systems, incorporated herein by reference, discloses the use of several different optical systems with collimating lenses, condenser lenses, cylinder lenses and telecentric lens systems, in a manner in which substantially more of the light from the light source from previous systems is directed past the component and collected for measurement allowing for a sharper component image on the detector. The use of these optical expedients allows the power requirements on a light source to be reduced by a factor of over one-hundred times over prior systems. With this reduction in required brightness levels, other light sources besides lasers such as light emitting diodes (LEDs) and incandescent bulbs can be used in component alignment.
The systems described by the '634 patent and the '279 application are well suited to the measurement of the orientation of the edges of the component. Furthermore, the entire sensing system can be integrated into the placement head. However, although angular orientation of the component can be achieved with an accuracy of better than 0.03 degrees and lateral positioning of the component can be achieved to an accuracy of better than 0.001 inches, these systems are not designed to precisely locate each of the leads of the component.
To detect any defects in the position of the leads, the light beam, in these systems, can be directed for some components at the leads projecting from the bottom of the component. The shadow cast by the leads is detected by a linear array detector whose data is analyzed to detect a leading edge and a trailing edge of the shadow. The shadow edge detection information is analyzed to obtain the lead position. But problems occur because the light beam is affected by leads on opposite sides of the component since the light beam is projected horizontally. Furthermore, the package standoff can be zero for some components. In that case, a horizontal beam cannot be used to measure the position of the leads because of interference of the beam by the body of the component. Since the system is optimized to measure the orientation of the body of the component, the system is designed to be horizontal within a very small tolerance, i.e. the light path is parallel to the seating plane.
While the prior sensor systems are suitable for handling all types and sizes of components, they have deficiencies either in speed, cost and/or accuracy. What is needed is an economical system which can accurately and rapidly locate leads having 50, 25, 20, 15, 10, and even 4 mil pitch. These systems must have a resolution in the range of at least 0.2 mils. Such a system would allow for much faster determination of lead position and coplanarity of the leads. A preferred system could combine all of these features in a single detection system and would allow a user to improve quality control at low cost, both in capital expenses and production times.